The present invention relates to high speed generators and, more particularly, to the structure of the rotors on such generators.
Generator systems that are installed in aircraft may include three separate brushless generators, namely, a permanent magnet generator (PMG), an exciter, and a main generator. The PMG includes permanent magnets on its rotor. When the PMG rotates, AC currents are induced in stator windings of the PMG. These AC currents are typically fed to a regulator or a control device, which in turn outputs a DC current. This DC current next is provided to stator windings of the exciter. As the rotor of the exciter rotates, three phases of AC current are typically induced in the rotor windings. Rectifier circuits that rotate with the rotor of the exciter rectify this three-phase AC current, and the resulting DC currents are provided to the rotor windings of the main generator. Finally, as the rotor of the main generator rotates, three phases of AC current are typically induced in its stator windings, and this three-phase AC output can then be provided to a load such as, for example, electrical aircraft systems.
Because the generators installed in aircraft will often be variable frequency generators that rotate in the speed range of 12,000 rpm to 24,000 rpm, large centrifugal forces are imposed upon the rotors of the generators. Given these stressful operating conditions, the rotors of the generators must be carefully designed and manufactured, both so that the rotors are reliable and also so that the rotors are precisely balanced. Improper balancing in particular not only can result in inefficiencies in the operation of the generators, but also potentially risk failures in the generators.
Among the important components in rotors that must be carefully designed and manufactured in order to guarantee reliability and proper balancing of the rotors are the wire coils of the rotors. The centrifugal forces experienced by the rotors are sufficiently strong as to cause bending of the wires of these coils, which over time can result in mechanical breakdown of the wires. Additionally, because the coils are assemblies of individual wires that can move to some extent with respect to one another and with respect to the remaining portions of the rotors, the coils constitute one of the significant potential sources of imbalance within the rotors. Even asymmetrical movements of these coils on the order of only a few thousandths of an inch can be significant.
In order to improve the strength and reliability of the wire coils and to minimize the amount of imbalance in the rotors that occurs due to the wire coils, wedges may be inserted in between neighboring poles of the rotors. The wedges in particular serve as physical barriers beyond which the wires of the coils cannot bend or move, and in many embodiments provide some pressure onto the coils that helps to maintain the physical arrangement of the coils.
Although the wedges employed in conventional rotors are capable of providing these benefits to some extent, the design of these conventional rotors and wedges limits the wedges"" effectiveness. Just as the wires of the coils of a rotor experience high centrifugal forces as the rotor rotates at high speeds, the wedges also experience high centrifugal forces. These forces tend to cause the wedges to spread radially outward away from the shaft of the rotor during operation, thus limiting the wedges ability to confine and place pressure upon the wire coils. Particularly, insofar as the axial lengths of conventional rotors are often relatively large in comparison with the diameters of the rotors, the centrifugal forces often tend to cause significant radial deflection or flexure of the wedges near their axial midpoints.
In order to prevent the wedges from spreading radially outward, many conventional rotors employ bands around the circumferences of the rotors to retain the wedges. In other conventional rotors, an xe2x80x9cunderwedgexe2x80x9d system is employed in which the wedges extend in their arc length all of the way between neighboring pole tips on the rotors, and snap rings are then employed to hold the wedges in place relative to the poles.
Yet these conventional structures for retaining wedges in place on rotors are limited in their effectiveness. Both the bands used to retain the wedges and the components of the underwedge systems (particularly the snap rings) also can suffer from bending during operation of the rotors. Because these devices suffer bending, the devices can only provide a limited amount of counteracting force to keep the wedges in place, and further can create additional imbalance in the rotors. Additionally, because it is difficult to accurately control the positioning of, and the amount of pressure applied by, the bands and underwedge componentry, it is difficult to accurately set and maintain the positioning of the wedges and to control the concentricity of the various wedges around the rotors.
Hence, there is a need for a new system and method for retaining wedges in a rotor. In particular, there is a need for a new system and method that allows for sufficient radial retention of the wedges of the rotor even at high speeds of operation, so that the wedges continue to provide support for and direct pressure towards the wire coils throughout operation of the generator. Further, it would be advantageous if the new system and method did not require components that had a tendency to bend in such a way as to create imbalance in the rotor. It would additionally be advantageous if the system and method allowed for the accurate positioning of wedges onto the rotor so as to provide concentricity of the rotor and its wedges. It would further be advantageous if the system was designed so as to allow the wedges to conduct heat away from the coils. It would additionally be advantageous if the system and method were relatively simple and inexpensive to implement.
The present inventors have recognized that conventional rotor wedges that are supported by bands or underwedge componentry near the circumference of a rotor can be replaced by two-wedge sets that each include an outer wedge and an inner wedge, where the inner wedges retain the outer wedges in their positions relative to the central axis of the rotor. The outer wedges, like conventional rotor wedges, expand in cross section as one moves radially outward from the shaft of the rotor. The outer wedges extend between neighboring poles of the rotor, and thereby provide support for, and direct pressure towards, the wire coils of those poles. The inner wedges are positioned radially inward from the corresponding outer wedges and are coupled to the outer wedges. The inner wedges expand in cross section as one moves radially inward toward the shaft of the rotor and rest upon the sides of the wire coils of neighboring poles. Consequently, the inner wedges are blocked from moving radially outward by the sides of the wire coils, and the sides of the wire coils provide the centripetal force necessary for restraining the inner and outer wedges in place. To the extent that any radial movement of the wedges does occur, the movement can only occur when accompanied by increased pressure applied on the coils. Further, the inner wedges and outer wedges can be coupled to one another by fastening devices that allow for variation in the relative positioning of the inner and outer wedges, and therefore allow for concentricity control.
In particular, the present invention relates to a rotor including a shaft extending along an axis through the rotor, first and second poles extending radially from the shaft, and first and second coils of wire windings respectively wrapped around the first and second poles. Each coil includes a respective outer face including two end turn portions and two side portions, and a respective inward-facing edge including two end turn sections and two side sections. The rotor further includes a first outer wedge positioned between a first of the side portions of the first coil and a first of the side portions of the second coil, and a first inner wedge positioned between a first of the side sections of the first coil and a first of the side sections of the second coil. The first inner wedge is coupled to the first outer wedge and provides support thereto so that the first outer wedge is at least partly retained from moving radially outward away from the shaft.
The present invention additionally relates to a high speed generator comprising a stator and a rotor, where the rotor includes a shaft extending along an axis through the rotor, a plurality of poles extending radially from the shaft, and a plurality of coils of wire windings. Each coil is wrapped around a respective one of the poles, and each coil includes a respective outer face formed by a respective outermost layer of wire windings of the respective coil and a respective pair of first and second edges. The respective outer face includes two end turn portions and two side portions, the first edge faces inward toward the shaft, the second edge faces outward away from the shaft, and each of the first and second edges includes two end turn sections and two side sections. The rotor additionally includes a plurality of outer wedges, where each outer wedge is positioned between a respective pair of the coils that neighbor one another so that the respective outer wedge is positioned between one of the side portions of a first coil of the respective pair and one of the side portions of a second coil of the respective pair. The rotor further includes a plurality of inner wedges, where each inner wedge is positioned between a respective pair of the coils that neighbor one another so that the respective inner wedge is positioned between one of the side sections of the first edge of a first coil of the respective pair and one of the side sections of the first edge of a second coil of the respective pair. The rotor additionally includes a means for coupling the respective inner wedges to the respective outer wedges.
The present invention further relates to an outer wedge for placement in a rotor assembly of a high speed generator. The outer wedge includes a main body that is substantially trapezoidal in cross-section and hollow. The main body additionally includes a pair of supports internal to the main body and respectively proximate each end of the main body. The outer wedge further includes a pair of end pieces, also having a substantially trapezoidal cross-section, where the end pieces are coupled to the supports by axial screws and further include holes by which coolant can be conducted through the end pieces.
The present invention additionally relates to an inner wedge for placement in a rotor assembly of a high speed generator. The inner wedge includes a main body having a substantially trapezoidal shape including a longer side, a shorter side, and two connecting sides. The main body is configured to be positioned between a pair of opposing side sections of a pair of coils of a pair of neighboring poles of the rotor assembly, and is configured to be coupled to an outer wedge.
The present invention further relates to a method of adjusting concentricity of a rotor in a high speed generator, where the rotor has coils of wire windings wrapped around respective poles of the rotor, and each coil includes a respective outer face having two end turn portions and two side portions. The method includes positioning inner wedges between opposing sides of the coils of neighboring poles, where the inner wedges have one or more first threaded openings. The method additionally includes positioning outer wedges between opposing side portions of the coils of neighboring poles, where the outer wedges have one or more second threaded openings collocated with the first threaded openings. The method further includes placing one or more threaded fasteners into collocated first and second threaded openings, and adjusting concentricity of the rotor by adjusting the threaded fasteners.
The present invention additionally relates to a method of retaining wedges of a rotor from moving radially outwards away from a shaft of the rotor during rotation of the rotor. The method includes wrapping coils of wire windings around respective poles of the rotor, where each coil includes a respective outer face including two end turn portions and two side portions, and further includes a respective inward-facing edge including two end turn sections and two side sections. The method additionally includes positioning inner wedges between opposing side sections of the coils of neighboring poles, positioning outer wedges between opposing side portions of the coils of neighboring poles, and coupling the outer wedges to the inner wedges through the use of fastening devices.
Other features and advantages of the high speed generator will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.